1. Field of Invention
This invention relates to a two speed gearbox and to a power tool incorporating such a gearbox.
2. Description of Prior Art
A known two speed gearbox incorporated in a nut running power tool is described in U.K. Pat. No. 808983. This known gearbox comprises a two speed epicyclic gear mechanism, a slipping clutch which transmits the higher speed drive and a free wheel or over-running clutch which automatically connects the spindle of the nut-engaging device with the lower speed drive when the first mentioned clutch slips. It has two major disadvantages. Firstly it cannot operate in reverse. This is a serious drawback in modern machine tools where it is often a requirement to tighten a nut to say about 80% of its final torque, and then undo the nut before running it down to its final torque. Secondly, the clutch members of the slipping clutch are never fully disengaged when the gearbox is operating at the lower of its two output speeds. This creates undesirable noise and damage to the clutch members.
U.K. Pat. No. 987,209 corresponding to U.S. Pat. No. 3,187,860, describes a two speed gearbox with a reduction gear which has a high speed low torque output drive and a low speed high torque output drive. A torque sensitive clutch connects the high speed output drive to an output shaft and includes a driven member in the form of a transverse pin which is movable along helical slots in the output shaft. A slidable coupling, rotatable with the output shaft and permanently engaged by the transverse pin, is movable between a first position in which it is disconnected from the low speed output drive and a second position in which it is connected to the drive in order to drive the output shaft. A compression spring releasably maintains the clutch in engagement and also maintains the slidable coupling in its first position while the clutch is engaged. The transverse pin running in the helical slots maintains the clutch fully out of engagement when the slidable coupling is in its second position. A disadvantage of this arrangement is that the pin is used both for torque transmission, at both speeds, and for moving the slidable coupling, and is therefore subject to heavy wear and high risk of failure. The gearbox cannot operate in reverse.